Fire Scene Evidence Collection Guide

Bodily Fluids

Touch DNA

Detailed Procedure:

Touch DNA is a term used to describe nuclear DNA that is transferred to an object simply by touching the object. The rougher the surface of the object is, the more skin cells are sloughed off or abraded, although there are other variables on the surface of the object that affect the transfer of DNA onto the surface. NOTE: Almost every piece of evidence has the potential for the presence of touch DNA.

To collect an item for touch DNA analysis:

  1. Wear new, unused, clean, and disposable latex or nitrile gloves. Wear respiratory protection as needed.

  2. Photograph the object with the suspected touch DNA.

  3. If the object is wet, place the item on new, clean, dry paper. Place the item and paper in a draft-free, dry location that is secure, ensuring that no one will touch, step on, remove, or displace the item. Allow the item to air dry. Do not package the item when wet; it may putrefy and therefore lose its evidentiary value.

  4. Label an appropriately-sized, air-permeable, and closable container, such as a cardboard box, paper envelope, or paper bag with identifying information, including case number, date, exhibit number, a brief description, and your name.

  5. When the item is dry, place it in the prepared container. Do not fold the item unless it is absolutely necessary.

  6. Seal the container with evidence tape, not your own saliva. Initial and date the tape.

  7. Make every effort to submit the entire item to be swabbed for touch DNA at the lab. However, if the item is an immovable surface, such as a tile floor or door knob, it can be swabbed at the scene using several different methods. Please contact your laboratory for instructions on how they would like the samples collected. One method that can be used is the double-swab method:

    1. Obtain 2 sterile swabs. Moisten 1 with sterile water. Swab the area with the moistened swab, and then go over the same area with the dry swab. Air dry both of the swabs in a position where the swabbed ends do not have contact with any surface.

    2. Before inserting the swabs in the container, label the container with identifying information, including case number, date, exhibit number, a brief description, and your name. Once the swabs are dry, place them cotton tip first into a swab box or a clean, new evidence collection envelope. Seal the envelope with tape, not your own saliva. Initial and date the tape.

    3. Submit a new, unused swab in a separate container as a control sample.

  8. Store the item in a secure, temperature-controlled location, such as a refrigerator or climate-controlled evidence storage room, until you are able to transport it to the laboratory. Do not leave the swabs or biological evidence in a hot car.

If the item will also be examined for ignitable liquid residue, please consult the laboratory for the proper collection procedure. Items should not be air-dried or packaged in an air-permeable container if they will be examined for ignitable liquid residue.

Laboratory testing of touch DNA:

Touch DNA samples can be examined to determine nuclear DNA profile (sex is determined during the DNA analysis), and other characteristics. Comparison of a questioned nuclear DNA profile to a known profile is possible. Comparison of DNA to other samples via computerized database (eg: CODIS for DNA profiles) may be possible.

Known DNA samples:

Generally, known DNA is collected using a buccal swab. Please contact your forensic laboratory for assistance. A court order may be required to collect this type of sample.

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